Magnetic Resonance Imaging (MRI) is one of the most important modern medical imaging modalities, with approximately 18 million procedures performed in 2001. MRI is currently used in three main areas: diagnostic imaging (dMRI), interventional imaging (iMRI), and quantitative imaging. Most commonly, it is used clinically for a variety of diagnostic procedures. These diagnostic procedures usually require high spatial resolution, high SNR, and low artifact levels. Additionally, it is important to have strong contrast between normal and pathological tissues. Available contrasts include: diffusion-weighted imaging for stroke; perfusion imaging for vascular tumors and infarction; spin-lattice (longitudinal) relaxation characteristic time constant T1 contrast; spin-spin (transverse) relaxation characteristic time T2 contrast; flow-sensitive imaging and MR angiography for detecting a variety of vascular pathologies, malformations and cardiac defects; and others.
Many of the background concepts and terminology used herein to describe or refer to MR imaging systems and their principles may be found, for example, in the book, titled “Magnetic Resonance Imaging: Physical Principles and Sequence Design,” by Haacke, et al., John Wiley & Sons (Wiley-Liss), 1999.
The raw data in MRI is acquired in k-space, which is the spatial frequency (also known as the Fourier domain) representation of the image or images of interest. In typical MRI data acquisition methods, k-space data are acquired—one line at a time—on a rectilinear (Cartesian) grid matrix. The image is then reconstructed, typically by using a fast Fourier transform (FFT) technique. However, rectilinear (Cartesian) data sampling methods are usually not time-efficient, and the reconstructed images may be affected by flow and motion artifacts. For this reason, some clinicians have proposed to use non-rectilinear (non-Cartesian) sampling paths, or trajectories, in k-space to acquire MRI data. One exemplary non-rectilinear trajectory is a spiral trajectory which permits acquisition of an MR image in 100 msec or less. Such spiral trajectories provide excellent immunity to flow artifacts and variable tissue contrast. However, since spiral trajectories sample only a limited number of points in k-space, such trajectories, if not properly selected, may introduce unwanted artifacts.
Most current k-space trajectory design techniques essentially begin with a trajectory shape that is easy to visualize and realize. The properties of the trajectory are examined and the real-space MRI image obtained from the various trajectories is then examined by a clinician for unwanted artifacts, and to ensure a faithful rendition of the tissue image. Two of the more common classes of non-rectilinear trajectories are spiral and radial. These two classes have several desirable properties, including rapid acquisitions and benign artifact patterns.
Most current techniques for designing k-space trajectories rely on the skill and expertise of clinicians, technicians, researchers, and other personnel, mostly employing heuristic rules derived from a collective available experience. In addition, in most cases, the clinician may not be able to ascertain the suitability of a trajectory without an unduly large number of trials, and may not be able to select or design a k-space trajectory that is optimized for more than one imaging parameter.